1. Field of the Invention:
The present invention relates to an optical harmonic generating device in which a harmonic wave of laser is generated from a fundamental wave of the laser and a shorter wavelength generating apparatus in which laser generated in a laser source is converted to a harmonic wave thereof by utilizing the optical harmonic generating device to produce coherent light utilized in a photo information processing field and a photo applied measuring control field.
2. Description of the Related Art:
An optical harmonic generating device has been conventionally developed to convert a fundamental wave of laser to a harmonic wave. The laser such as semiconductor laser is generally composed of the fundamental wave and a plurality of harmonic waves such as a second harmonic wave. The intensity of the laser is almost occupied by the fundamental wave. A wavelength .lambda..sub.f of the fundamental wave is longer than those of the harmonic waves, and each of wavelengths .lambda..sub.h of harmonic waves is defined as a multiple of the wavelength .lambda..sub.f. For example, the wavelength .lambda..sub.h of the second harmonic wave is half of the wavelength .lambda..sub.f. Therefore, the conversion of the fundamental wave to the harmonic wave means that the wavelength of the fundamental wave is shortened.
The harmonic wave is generally produced in a shorter wavelength generating apparatus. That is, a fundamental wave of laser generated in a laser source is converted to a harmonic wave in the shorter wavelength generating apparatus. Because the harmonic wave has a shortened wavelength, the harmonic wave is useful in a photo information processing field. That is, the harmonic wave is utilized as a data signal, and information consisting of a large number of data signals is processed in the field. Also, the harmonic wave is useful in a photo applied measuring control field. That is, a length of a subject is measured by utilizing the harmonic wave.
2.1. Previously Proposed Art:
A conventional optical harmonic generating device is described with reference to drawings. For a detailed description on the conventional optical harmonic generating device, see E. J. Lim, M. M. Fejer, R. L. Byer and W. J. KoZlovsky, "Blue Light Generation by Frequency Doubling in Periodically-Poled Lithium Niobate Channel Waveguide", Electronics Letter, Vol. 27, P731-732, 1989.
FIG. 1 is a diagonal view of a conventional optical harmonic generating device.
As shown in FIG. 1, a conventional optical harmonic generating device 11 is provided with a substrate 12 made of non-linear optical crystal LiNbO.sub.3 which is dielectrically polarized in an upper direction, a plurality of reverse polarization layers 13 which are periodically arranged in an upper side of the LiNbO.sub.3 substrate 12 at regular intervals and are dielectrically polarized in a lower direction, a plurality of non-reverse polarization layers 14 which are periodically formed in the upper side of the LiNbO.sub.3 substrate 12 between the reverse polarization layers 13 and are dielectrically polarized in the upper direction, and a wave guide 15 for guiding a fundamental wave P1 of laser from one side of the LiNbO.sub.3 substrate 12 to another side through the non-reverse polarization layers 14 and the reverse polarization layers 13.
The reverse and non-reverse polarization layers 13, 14 are also made of the non-linear optical crystal LiNbO.sub.3.
The wave guide 15 penetrates alternate rows of the non-reverse and reverse polarization layers 13, 14.
In the above configuration, an operation of the conventional optical harmonic generating device 11 is described.
A beam of laser almost occupied by a fundamental wave P1 of which a wavelength is 820 nm is radiated from a laser source (not shown) to one side of the wave guide 15. The fundamental wave P1 radiated to one side of the wave guide 15 transmits through the waveguide 15. At this time, because the fundamental wave P1 passes through the alternate rows of the reverse and non-reverse polarization layers 13, 14, the fundamental wave P1 is converted to a second harmonic wave P2 of which a wavelength .lambda..sub.h is 410 nm in the reverse polarization layer 13. The phase of the harmonic wave P2 converted is inverted while passing through the reverse polarization layer 13. Thereafter, the harmonic wave P2 of which the phase is inverted passes through the non-reverse polarization layer 14. In this case, because the polarization direction of the non-reverse polarization layer 14 is opposite to the reverse polarization layer 13, the harmonic wave P2 passing through the non-reverse polarization layer 14 is amplified without attenuating. Therefore, the second harmonic wave P2 is radiated from the another side of the wave guide 15 in place of the fundamental wave P1.
Accordingly, mismatching between a propagation constant of the fundamental wave P1 and a propagation constant of the harmonic wave P2 can be compensated by a periodical structure consisting of the alternate rows of the reverse and non-reverse polarization layers 13. As a result, the harmonic wave P2 can be efficiently amplified.
FIG. 2 graphically shows an intensity variation of the second harmonic wave P2 radiated from the another side of the wave guide 15, in comparison with another intensity variation of another harmonic wave P3 radiated from another wave guide 16 in which the reverse polarization layers 13 are replaced with the non-reverse polarization layers 14.
As shown in FIG. 2, in cases where the reverse polarization layers 13 are replaced with the non-reverse polarization layers 14 in another wave guide 16, the intensity of another harmonic wave P3 is periodically increased and decreased at the regular intervals of the non-reverse polarization layers 14 even though a length L of the wave guide 16 is prolonged. Accordingly, the harmonic wave P3 are not amplified.
In contrast, in cases where the alternate rows of the reverse and non-reverse polarization layers 13, 14 are periodically arranged in the wave guide 15 at the regular intervals, the intensity of the harmonic wave P2 is increased in proportion to a squared value L.sup.2 of the length L of the wave guide 15.
In this case, a pseudo-phase matching condition is satisfied to amplify the harmonic wave P2. The pseudo-phase matching condition is formulated by an equation. EQU .LAMBDA.1=.lambda..sub.f /{2*(N2.omega.-N.omega.)} (1).
Wherein, the symbol A1 denotes a pitch between the reverse polarization layers 13, the symbol .lambda..sub.f denotes a wavelength of the fundamental wave P1, the symbol N2.omega. denotes an effective refractive index of the wave guide 15 for the harmonic wave P2 of the wavelength .lambda..sub.h =.lambda..sub.f /2, and the symbol N.omega. denotes an effective refractive index of the wave guide 15 for the fundamental wave P1 of the wavelength .lambda..sub.f. The effective refractive index of the wave guide 15 is defined as an average value of those of the reverse and non-reverse polarization layers 13, 14.
Next, a method for manufacturing the conventional optical harmonic generating device 11 is described with reference to FIGS. 3A to 3C.
FIGS. 3A to 3C are respectively a sectional view of the LiNbO.sub.3 substrate, showing processes for manufacturing the conventional optical harmonic generating device 11.
As shown in FIG. 3A, a patterned film 17 made of Ti is deposited on the LiNbO.sub.3 substrate 12 according to a lift-off process. In this case, a plurality of rectangular openings 18 surrounded by the Ti film are formed at the pitch A1 on the LiNbO.sub.3 substrate 12. The pitch A1 is several micrometers.
Thereafter, as shown in FIG. 3B, surfaces of the LiNbO.sub.3 substrate 12 exposed to the openings 18 are heated at a temperature of almost 1050.degree. C. Therefore, the LiNbO.sub.3 substrate 12 exposed to the openings 18 is changed to the reverse polarization layers 13. Thereafter, The Ti film 17 is taken off.
Thereafter, as shown in FIG. 3C, the LiNbO.sub.3 substrate 12 with the reverse polarization layers 13 is immersed in a hot benzoic acid and is heated for twenty minutes at a temperature of 200.degree. C. After the LiNbO.sub.3 substrate 12 is taken up from the hot benzoic acid, the surface of the LiNbO.sub.3 substrate 12 is annealed for three hours at a temperature of 350.degree. C. so that the wave guide 15 is produced in the LiNbO.sub.3 substrate 12.
Because the LiNbO.sub.3 substrate 12 is immersed in a hot benzoic acid, an electric power of the harmonic wave P2 is 940 nW on condition that the length of the wave guide 15 is 1 mm and an electric power of the functional wave P1 is 14.7 mW.
2.2. Problems to be solved by the Invention:
However, a half band width of the fundamental wave P1 is only 0.1 nm on condition that the length of the wave guide 15 is 5 mm. That is, as shown in FIG. 4, in cases where the wavelength .lambda..sub.f of the fundamental wave P1 varies by 0.05 nm, the electric power of the harmonic wave amplified in the conventional optical harmonic generating device 11 is reduced to half of a maximum value. Therefore, the permissible variation of the wavelength .lambda..sub.f of the fundamental wave P1 is small.
Also, the wavelength of semiconductor laser generated in a semiconductor laser source is easily changed in cases where the temperature of the semiconductor laser source is changed according to environmental conditions such as ambient temperature.
Accordingly, in cases where semiconductor laser generated in the semiconductor laser source is radiated to the conventional optical harmonic generating device 11 in a conventional shorter wavelength generating apparatus, the harmonic wave P2 is not radiated from the device 11, or the electric power of the harmonic wave P2 is considerably reduced because the temperature of the semiconductor laser source is changed.
For example, as shown in FIG. 4, in cases where the wavelength of the semiconductor laser almost occupied by the fundamental wave P1 is 820 nm, the electric power of the harmonic wave P2 is maximized because the pseudo-phase matching condition is completely satisfied. Also, in cases where the temperature of the semiconductor laser source is 20.degree. C., the wavelength of the semiconductor laser is maintained at a value 820 nm. However, in cases where the temperature of the semiconductor laser source is changed from 20.degree. C. to 21.degree. C., the wavelength of the semiconductor laser is changed from 820 nm to 820.2 nm. Therefore, the electric power of the harmonic wave P2 becomes zero.